1. Field of the Invention
The present invention relates to a magnetic recording head that has a spin torque oscillator suitable for data storage with a high recording density, a high recording capacity, and a high data transmission rate, and a magnetic recording device equipped with the magnetic recording head.
2. Related Art
In the 1990's, the recording density and capacity of HDDs (Hard Disk Drives) dramatically increased, with MR (Magneto-Resistive effect) heads and GMR (Giant Magneto-Resistive effect) heads being put into practical use. However, the problems of thermal fluctuations of magnetic recording media became apparent in the early 2000's, and the increase of the recording density temporarily slowed down. In 2005, perpendicular magnetic recording that was more suitable for high-density recording in principle than for longitudinal magnetic recording was put into practical use. This invention has become the driving force and since then, the HDD recording density has been increasing at an annual rate of approximately 40%.
The latest examinations on the recording density show that the recording density of 400 Gbits/inch2 has been reached. If the growth continues strong at this rate, the recording density of 1 Tbits/inch2 will be achieved around the year 2012. However, achieving such a high recording density is not easy by the perpendicular magnetic recording method, because the problem of thermal fluctuations rearises.
To counter this problem, a “microwave assisted magnetic recording method” has been suggested. By the microwave assisted magnetic recording method, a high-frequency magnetic field at a frequency in the neighborhood of the resonant frequency of a magnetic recording medium, which is much higher than the recording signal frequency, is locally applied. As a result, the magnetic recording medium resonates, and the coercivity Hc of the magnetic recording medium having the high-frequency magnetic field applied thereto decreases to half the original value. Therefore, a high-frequency magnetic field is overlapped with the recording magnetic field, so that magnetic recording can be performed on a magnetic recording medium having higher coercivity Hc and greater magnetic anisotropic energy Ku (see U.S. Pat. No. 6,011,664, for example). However, according to U.S. Pat. No. 6,011,664, a high-frequency magnetic field is generated with a coil, and it is difficult to efficiently apply a high-frequency magnetic field at the time of high-density recording.
To counter this problem, a method of utilizing a spin torque oscillator as the means of generating a high-frequency magnetic field has been suggested (see United States Patent Application Publication No. 2005/0023938, for example). According to United States Patent Application Publication No. 2005/0023938, the spin torque oscillator is formed with a spin injection layer, a nonmagnetic layer, a magnetic layer, and an electrode layer. When a direct current is applied to the spin torque oscillator through the electrode layer, the magnetization of the magnetic layer has ferromagnetic resonance due to a spin torque generated from the spin injection layer. As a result, a high-frequency magnetic field is generated from the spin torque oscillator.
Since a spin torque oscillator is several tens of nanometers in size, a high-frequency magnetic field is generated at a proximity of several tens of nanometers from the spin torque oscillator. Accordingly, a perpendicular magnetic recording medium can resonate effectively by virtue of the in-plane component of the high-frequency magnetic field, and the coercivity of the magnetic recording medium can be greatly reduced. As a result, high-density magnetic recording can be performed only on the overlapping portion between the recording magnetic field generated from the main magnetic pole and the high-frequency magnetic field generated from the spin torque oscillator, and a magnetic recording medium with high coercivity Hc and large magnetic anisotropic energy Ku can be used. Thus, the problem of thermal fluctuations during a high-density recording operation can be avoided.
In a high-frequency assisted recording operation, however, the recording magnetic field generated from the main magnetic pole is applied to the spin torque oscillator, and the frequency of the high-frequency magnetic field generated from the spin torque oscillator fluctuates. Because of this, a high-frequency magnetic field having a constant frequency cannot be obtained, and stable magnetic recording cannot be performed.